Catalyst compositions of scheelite crystal structure containing bismuth ions and cation vacancies

ABSTRACT

CATALYSTS FOR THE OXIDATION, AMMOXIDATION AND OXIDATIVE DEHYDROGENATION OF OLEFINS COMPRISE COMPOSITIONS HAVING SCHEELITE TYPE CRYSTAL STRUCTURE, BISMUTH, AND CATION VACANCIES. ILLUSTRATIVE IS NA.44BI.52$.04MO O4.

United States Patent No Drawing. Continuation-impart of abandonedapplication Ser. No. 75,237, Sept. 24, 1970. This application Oct. 4,1971, Ser. No. 186,408

Int. Cl. B01j 11/08 U.S. Cl. 252--462 12 Claims ABSTRACT OF THEDISCLOSURE Catalysts for the oxidation, ammoxidation and oxidativedehydrogenation of olefins comprise compositions having a scheelite typecrystal structure, bismuth, and cation vacancies. Illustrative is Na- Bi\:l M0O RELATED APPLICATION This application is a continuation-in-partof our application Ser. No. 75,237, filed Sept. 24, 1970, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention The invention relatesto catalytic oxidation and ammoxidation of olefins, as for example, theconversion of propylene to acrolein or to acrylonitrile. The catalystshave a scheelite crystal structure and contain bismuth ions and cationvaccines.

Prior art Catalysts for the oxidation and ammoxidation of olefins areknown in the art and include materials based on bismuth molybdate wherethe bismuth is present above a definite minimum ratio with respect tothe molybdenum. The art generally teaches the amount of bismuth shouldnot be less than /3 the amount of molybdenum and preferably the ratioshould be 3:4, or more, in order to avoid sublimation of the molybdenumand consequent decomposition of the catalyst. Such catalysts havedisadvantages in being relatively brittle and thermally unstable.Austrian Pats. 247,304 and 248,410 seek to overcome these disadvantagesby combining bismuth and molybdenum oxides with divalent metals such ascalcium and lead. Koch Pat. U.S.. 3,387,038 teaches that molybdenumoxide is eflective as a catalyst when combined solely with alkalineearth oxides although the catalytic effect is further promoted by minoramounts of many elements including bismuth. Even the preferredembodiments of Kochs catalysts, however, give relatively low conversionsof propylene to acrolein and require the periodic addition of molybdicacid in order to prevent the loss of that activity.

Silicon and phosphorus are widely recognized as useful promoters forbismuth-molybdenum oxide compositions used as catalysts. This isbelieved to be related to the well known tendency of these two elementsto combine in mixed oxides (heteropoly acids) with molybdenum dtungsten. Silica can be intimately incorporated by combining thecatalyst precursors in the presence of a colloidal silicic acid. Eventhough such silica may be termed a support, its special interaction isrecognized by Callahan et al., U.S. 3,362,998 and McClellan, U.S.3,415,886, and auxiliary supports are generally used to support thesesilica-bearing catalysts during reaction.

3,806,470 Patented Apr. 23, 1974 DESCRIPTION OF THE INVENTION It has nowbeen found that compositions having a phase, generally indicated by theformula ABO of scheelite-type crystal structure in which some of the Acation sites are vacant, that is, not occupied by any ion, and some ofwhich are occupied by bismuth ions, are effective catalysts for theammoxidation of olefins to unsaturated nitriles, the oxidation ofolefins to unsaturated aldehyde, and the oxidative dehydrogenation ofolefins to diolefins. The number of the A sites that are either cationvacancies (represented hereinafter by the symbol [1) or occupied bybismuth ions may be very small but these two types of site occupationmust coexist for the composition to have high catalytic activity.

It has been further found that the remaining A cation sites, and the Bcation sites, may be occupied by a variety of positive ions ofappropriate size, that is, having ionic radii which will notsubstantially change the scheelite crystal structure. A large number ofions may be selected provided the electrical charge of the compositionis zero.

The ions occupying the type A sites of the scheelitetype crystalstructure generally are coordinated by eight surrounding oxygen atomsand have ionic radii appropriate to this coordination in the range ofabout 0.9 to about 1.6 A. According to the invention the type A cationsmay be selected from the group consisting of bismuth, silver, sodium,lithium, potassium, zirconium, hafnium, yttrium, lanthanides (elementshaving an atomic number in the range 57-71), uranium, thallium andthorium. 'In addition to bismuth, which must be present, preferred typeA cations are sodium, lithium, silver and yttrium. As previouslyindicated, it is also essential that some type A cation sites remainunoccupied.

The ions occupying the type B sites of the scheelitetype crystalstructure are generally tetrahedrally coordinated by oxygen and haveionic radii appropriate to this coordination ranging from about 0.3 toabout 0.5 A. According to the invention, the type B cations are selectedfrom the group consisting of molybdenum, tungsten, iron, germanium,zinc, arsenic, rhenium, gallium, aluminum, niobium and chromium.Molybdenum, tungsten and iron are preferred type B ions. Especiallypreferred are compositions in which the B ions consist of molybdenum andiron.

It is preferred for maximum effectiveness that the scheelite-typecatalyst consist essentially of a single phase.

A further preferred embodiment of the invention, because of theirexcellent catalytic activity are the compositions ABO, in which up toabout 15% of the A cation sites are vacent.

The invention may thus be described as a catalyst which hasscheelite-type crystal structure of the general formula wherein Arepresents cations having ionic radii in the range of about 0.9 to about1.6 A., some of which are trivalent bismuth, and others are optionallyselected from the group consisting of silver, sodium, lithium,potassium, zirconium, hafnium, yttrium, lanthanides, uranium, thalliumand thorium; B represents one or more cations having ionic radii in therange of about 0.3 to about 0.5 A. selected from the group consisting ofmolybdenum, iron, germanium, zinc, arsenic, rhenium, gallium, aluminum,niobium and chromium; and z is a positive number up to about 0.15expressed as 0 z 0.l5.

Scheelite-type crystal structure means a phase of generic type ABO, withan atomic arrangement analogous to that found in the mineral scheelite(CaWO A discussion of scheelite crystals may be found in R. W. G.Wyckoif, Crystal Structures, vol. 3, 2nd ed. 1965, pp. 19-22, publishedby Interscience Publishers. Scheelite crystal structures generally havetetragonal symmetry and can be characterized by the two latticeconstants a and c as obtained by X-ray diffraction data. The range of ais about 4.8 to 6.0 A. and the ratio of c /a is about 2. The atomicarrangement in the scheelite structure, also given by Wyckoif, givesrise to a characteristic X-ray diffraction pattern by which materialshaving scheelite crystal structure can be identified.

It is to be understood that scheelite-type crystal structure is intendedhere and in the claims to also cover variant crystal structures whichhave minor distortions in angle or edge size from the usual or classicaltetragonal symmetry given above. For example, when the distortionresides in a third lattice constant, b an orthorhombic distortionresults while a distortion in either angle 3 or 7 results in amonoclinic distortion. Where fi is within 10% of the value of d eachbeing in the range of about 4.8 to 6.0 A., and where the angle 5 or 'yis within 5 of the normal 90 angle, such crystal structures arecontemplated to be within the scope of scheelite-type crystal structuresof this invention. It should also be noted that a and 0 or b and may beinterchanged to conform to certain crystallographic conventions and inthe case of monoclinic symmetry the unit cell may be redefined so that18 or 'y departs greatly from 90. In all the variations which may occur,however, the essential atomic arrangement is present which determinesand characterizes scheelite-type crystal structure. That atomicarrangement is identified by the characteristic X-ray diffractionpattern it produces even though some of the diffraction peaks will besplit in the case of the variant crystals.

It will also be recognized that where a single type of cation site isoccupied by more than one cation then under some conditions nonrandomsite occupation will produce superstructure lines in the characteristicscheelite X-ray pattern. Under such circumstances the exactcharacterization by unit cell parameters will require multiplication ofone or more of the unit cell dimensions by a small whole number in orderto characterize the special proportionate distribution of cations. Thus,for example, a defect-free scheelite-type structure might be producedfrom the stoichiometry BiFe Mo O in which /3 of the type B sites areoccupied by Fe+ ions. The arrangement of Fe+ and Mo+ ions, however, isapparently an ordered one since weak superstructure lines appear in theX-ray pattern, and these require a larger unit cell for indexing. Theresultant monoclinic cell a=16.16 A. (equivalent to 3 5.39 A.), b=5.25A., 0:11.65 A. and =90.97, larger by extension along the a axis, isreadily seen to be a mere ordering modification of the monoclinicscheelite variant within the limits previously given for scheelites. Onthe other hand, the well-known bismuth molybdate Bi O -3Mo0 does nothave a scheelite-type structure within the meaning given here as can beseen by its distinctly different X-ray pattern.

The charge on the various A and B metal ions can be varied consistentwith the requirement that the sum of the positive charges of all A and Bmetal ions is exactly balanced by the sum of the charges on the negativeions. The generic formula ABO for scheelite thus includes ternary oxidesranging from as well as polynary oxides where either A or B or both maycomprise a mixture of ions of appropriate size whose average charge iscovered by the range above.

The presence of bismuth ions on the type A cation sites can bedetermined by customary analytical procedures for the determination ofbismuth since its ions are too large for the type B sites. In eight-foldcoordination Bi has an ionic radius of 1.11 A. and is well suited fortype A sites.

The presence of unoccupied type A cation sites can be determined by adeficiency of type A cations with respect to type B metal ions. Since inthe scheelite-type structure there is one A and one B site per formulaABO and since the B cation sites are always fully occupied by the type Batoms, the number of type A cation vacancies, z, can be determined bysubtracting the number of type A atoms from the number of type B atoms.Thus in the singlephase scheelite-type composition whose formula hasbeen determined by elemental analysis as Nail Bit; Mo 0 the sum of thetype A gram atoms is 0.96. Since there is one B type atom (molybdenum)present in the above formula, it follows that the sum of the two type Aatoms (sodium and bismuth) plus any type A site vacancies, must equal 1.As noted, however, the type A atoms which are present total only .96which indicates there are .04 type A sites that are vacant. The formulafor the above single phase scheelite-type composition is therefore moreproperly written as This composition is electrically neutral since the 8negative charges from oxygen are just balanced by 6 positive chargesfrom molybdenum, 1.56(3 .52) from Bi, and .44 from Na for a total of 8positive charges.

The E0; tetrahedra are characteristic of the scheelitetype structureeven though a certain amount of type A cations which link adjacenttetrahedra may be missing from the lattice. Thus, oxides of thescheelite phase will have, within normal analytical accuracy, fouroxygen anions per B cation. In preparing scheelite catalysts of theinvention the component oxides or their oxide precursors should 'bechosen so that in their normal oxidation states after calcining, ashereinafter described, four gram atoms of oxygen will be present foreach gram atom of B cation. The presence of substantially more or lessoxygen relative to the B cation will lead to dilution of the AB0catalyst of the invention with another phase. The proper proportions ofthe various oxides or oxide precursors that should be combined to form apure scheelitetype phase of desired vacancy content can be readilydetermined from the requirements of electroneutrality and siteoccupation described above. In the simple cases where a single ionoccupies the B sites and the necessary cation vacancies result fromsimple permutation of the A cation charge, the following genericformulae pertain for ternary oxides, where z equals the number ofvacancies:

l 7513. 1 04 l 7215. Eiz 1 04 22 1 4, I]. 04

The necessary type A cation vacancies can also be provided bypermutations of variously charged ions on the type B cation sites as inthe system For example, to prepare a pure scheelite-type catalyst in theternary system Bi O -Fe O -MoO having a cation vacancy content of 4%,the last formula above would be used with z=0.04 to give the preferredmolar proportions of starting materials, viz., 0.480 Bi O 0.146 Fe O0.707 M00 It will be understood that further permutations of both type Aand type B cations in the same composition can be devised to provide avery wide range of new catalytic materials with specified content of Bi+and vacancies on the type A cation sites.

It should be kept in mind that when the number of defects or cationvacancies becomes too high, their random occurrence may be replaced byan ordered arrangement with the production of a crystal structure thatdeparts from the limits hereinbefore described for the scheelite-typestructure. It is to be expected that the change from the scheelite-typestructure to other phases with ordered vacancies will depend in acomplex way on the temperature and on the particular ions in thelattice. It has been found that in many systems corresponding to theformula and Bi+ ions coexist. However, it is desirable for maximumeffectiveness to prepare the catalyst compounds as substantially purephases. These are characterized by X-ray diffraction powder diagramswhich can be indexed in their entirety according to the scheelitestructure. Large amounts of excess or unreacted components will act asdiluents and can interfere with the intended catalytic function,particularly if they accumulate at the surface.

The catalysts of the invention may thus be characterized in that:

(1) There are four oxygen atoms for each B atom.

(2) The type A and type B ions are selected from those cationsappropriate to the scheelite-type structure as indicated herein.

(3) They are electrically neutral, the number of positive chargesexactly equaling the number of negative charges.

(4) The type A sites, equal in number to the type B sites, include siteswhich are unoccupied and sites which are occupied by trivalent bismuth.

(5) Their X-ray diffraction pattern is characteristic of scheelite-typecrystal structure.

The catalysts of the invention can be prepared by various methods. Theusual method is to calcine mixtures of oxides, or any salt which yieldsthe corresponding oxide by thermal decomposition, e.g., carbonates,nitrates, oxalates, hydroxides, etc., in the proper ratios for thedesired composition. The catalysts can also be prepared by mixingsolutions containing proper amounts of the type A and type B metalsalts, evaporating, drying, and finally calcining. The startingsolutions are usually nitrates in the case of A, A+ A and Bi, but othersoluble salts can be used. The solvent can be water or an organic liquiddepending on the solubility of the salt. The amount of solvent is notcritical and it is preferred to use only that amount needed to effectcomplete solution of the salt.

Calcining temperatures vary from about 400 to about 1100 C. The optimumtemperature depends upon the particular catalyst being prepared. Highertemperatures are preferred in order to facilitate reaction and to assurea homogeneous poduct. A critical upper limit for calcining temperaturesis imposed by the formation of a liquid phase. The products of theinvention have incongruent melting points, i.e., the liquid and solid inequilibrium in the melting range have quite different compositions. Aliquid phase formed during calcining tends to produce both compositionaland physical segregation which are diflicult to homogenize at lowertemperature. Higher temperatures are favored in general for catalystscontaining tungsten while lower temperatures are used for catalysts richin bismuth and molybdenum.

The time of calcination is not critical; times of 1 to 100 hours may beused, but 16-48 hours is preferred. Longer times are required at lowertemperatures. Calcining times may be shortened and homogeneity of theproducts improved by regrinding between periods of heating. A generallyadvantageous procedure is to grind the dry components intimately,calcine for 2-16 hours at 600- 800 C., then regrind and calcine againfor 16-32 hours at 600-800 C.

The container used for calcining may be made of various inert materialssuch as gold or other precious metals, alumina or other ceramics. Thecalcination is usually carried out in a muffle furnace in which thesample is exposed to an atmosphere of air. Reducing atmospheres shouldbe avoided to prevent the reduction of oxides of easily reduced metalssuch as silver and bismuth.

The completeness of the reaction may be followed by X-ray diffraction ofthe products at any stage. When formation of a cation-deficientscheelite-type phase is complete, all lines of the X-ray powder patterncan be indexed on the basis of a scheelite-type unit cell with. lineintensities in qualitative agreement with those expected for thescheelite-type structure. The dimensions of the unit cells of thesescheelite phases vary significantly with changes in either type ornumber of cations in the A sites, and can be used to characterize thecomposition of the phase obtained. For example, in the tetragonalscheelite-type system equilibrated at 600 C., the cell length, cincreases regu larly from 11.469 A. at z=0 to 11.627 A. at z=0.15;further attempts to increase z do not affect the cell dimensions butbegin to introduce extraneous X-ray lines. While somewhat differentlimits may be found at other tempera tures and with other ions, thepresence of a cation deficient, Bi-containing scheelite-type structureis shown by the significant difference in the cell dimensions from thoseof the defect-free phases.

The products of the invention are excellent hetero geneous catalysts fororganic oxidation reactions. They are particularly useful for theoxidation of propylene to acrolein, the production of acrylonitrile frompropylene, NH and O and the conversion of butene to butadiene. Thecatalysts show excellent conversions of propylene, good selectivity, andretain their high initial activity without need for frequent and costlyregeneration steps. The presence of water in the feed gas is notnecessary as it is with many prior art catalysts, but it may be used ifdesired. Air or oxygen may be used, or the feed gas may be furtherdiluted with nitrogen, for example. The catalyst may be used infixed-bed or fluidized-bed reactors; they may be used with most of theusual catalyst support materials or they may be used without support,and any type of reactor suitable for vapor phase reactions may beemployed.

The temperature of the reaction zone may vary from 350 C. to 550 C.,although it is preferred to operate within the temperature range of 400C. to 500 C. The actual surface temperature of the catalyst particlesmay be considerably higher because of the exothermic nature of thereaction. Pressure is not a critical factor in the practice of thisinvention. The process may be conveniently operated at atmosphericpressure. In most instances, the reaction is conducted at pressuresranging from 0.5 to 10 atmospheres, but higher or lower pressures may beused if desired.

The oxygen used in this process may be obtained from any source,although it is generally most economical and convenient to use air.Alternatively, pure oxygen or mixtures of oxygen and air may be employedin the oxidation process, including oxidative dehydrogenation process,and a mixture of air and ammonia for the ammoxidation process.

Neither the presence of cation vacancies alone nor the presence ofbismuth ions alone is sufiicient to produce good catalytic activity inthe scheelite-type compositions of the invention. However, when bothcoexist, even in small amounts, excellent catalytic activity isobtained.

The use of very high surface area or reactive materials as catalystsupports is to be avoided. In particular, intimate mixtures of thecation deficient scheelite-type compositions of the invention withreactive materials such as silica should not be excessively heated.

It will be obvious to one skilled in the use of catalysts for carryingout oxidation reactions that the composition of the catalytic sites onthe catalyst surface during the course of the reaction may departsomewhat from initial stoichiometry. Particularly in oxidation reactionselements of variable valency may coexist in more than one oxidationstate as an essential feature of the catalytic mechanism. Thus in thepresent invention it is contemplated for example that M0 or W may bepresent to a small extent as pentavalent species along with thepreponderant hexavalent species. Similarly, bismuth could exist to somesmall extent in an oxidation state greater than in the normal trivalentcation. Such variations can be equivalently represented as slightdepartures from the oxygen stoichiometry hereinbefore described. Thus inthe idealized A'BO formula there might be slightly more or less than4.00 g. atoms of oxygen, or a small fraction of the ions might bereplaced by OH" ions without departing from the spirit of the invention.

By the term olefin as used herein is meant the openchain as well ascyclic olefins. Among the many olefinic compounds which may be utilizedin accordance with the process of the invention, the following compoundsare illustrative; propylene, butene-l, butene-Z, isobutylene, pentene-l,pentene-2, 3-methylbutene-l, Z-methyl-butene- 2, hexene-l, hexene-Z,4-methyl pentene-l, 3,3-dimethylbutene-l, 4-methyl-pentene-2, octene-l,cyclopentene, cyclohexene, 3-methyl-cyclohexene, etc. This invention isdirected particularly to the oxidation of the lower alkenes (3 to 8carbon atoms) but higher alkenes may also be utilized with eflicacy.These compounds and their various homologs and analogs may besubstituted in the nucleus and/or in the substituents in various degreesby straightchain alicyclic or hetero-cyclic radicals. The process of theinvention is applicable to individual olefins as well as to mixtures ofolefins.

The process of this invention is particularly adapted to the conversionof propylene to acrolein, isobutylene to methacrolein, butene-l orbutene-2 to methyl vinyl ketone, pentene-1 or pentene-2 to ethyl vinylketone and/ or pentene-3-one-2, 2-methyl-butene-2 to methyl isopropenylketone, cyclopentene to cyclopentenone-Z, and the like.

SPECIFIC EMBODIMENTS OF THE INVENTION The following are illustrativeexamples in which all parts or percentages are by weight unlessotherwise stated.

EXAMPLE 1 2.332 g. of Na CO 12.116 g. of 'Bi 0 and 14.395 g. of M00 aremixed by grinding in a mortar, calcined in air for 16 hours at 625 C.,reground, and calcined an additional 16 hours at 625 C. The product is asinglephase tetragonal scheelite-type structure with the latticeparameters, a =5.276 A. and c =11.595 A. A theoretical density on thebasis of X-ray data for the composition Na Bi D MoQ, is 5.74 g./cm. incomparison to its observed density of 5.75 g./cm. by the pycnometermethod.

Using the procedures given in this specification the following productsof the invention are obtained.

TABLE I EXAMPLE 2 27.595 g. of single-phase tetragonal Na Bi MoO ismixed with 14.964 g. of single-phase, monocline by ball-milling and isthen calcined in air for 16 hours at 640 C. It is reground and calcinedan additional 16 hours at 640 C. By X-ray difiraction analysis theproduct is a pure tetragonal scheelite-type phase. The composition basedon starting material is 8 EXAMPLE 3 A solution of 27.20 g. of NaNO in 43ml. B 0 is mixed with a solution of 271.66 g. of Bi(NO -5H O in amixture of 19.4 ml. conc. HNO and 194 m1. H O. A solution of 176.56 g.of (NH4)6M07O24'4H2O in 200 ml. H O is added with vigorous agitation.The above mixture is homogenized by ball-milling, evaporated on a steambath, dried for three hours at about C., calcined in air for 16 hours at600 C., reground, and calcined for an additional 16 hours at 625 C. Theproduct is a single-phase tetragonal scheelite-type structure with aformula of Na Bi Mo0 and with the lattice parameters, a =5.276 A. and c=11.640 A.

A portion of this product was mixed with an equal weight of SiC powderand formed into A;" pellets for testing catalytic activity in a V2" I.D.fixed bed reactor. Using a feed gas composed on a molar basis of 5.0%propylene, 47.9% air, and 47.1% nitrogen for a 2.0 second contact timeat 485 C., 83.5% of the propylene was consumed and 61.0% of the initialpropylene was converted to acrolein. When the feed gas consisted of 4.0%propylene, 4.9% ammonia, 48.7% air, and 42.4% nitrogen, then 88.4% ofthe propylene was consumed and 64.6% of the initial propylene wasconverted to acryloni trile while only 1.8% was converted to acrolein.

A similar composition but free of cation vacancies, Na Bi [:|MoO wasprepared and tested under similar conditions to those descibed above.Only 5.2% and 2.7% of the propylene feed was consumed under conditionsfor making acrolein and acrylonitrile, respectively.

Another portion of the composition mixed with an equal weight of SiCpowder was tested in the same reactor using as feed gas on a molar basis5.0% l-butene, 26.0% air, and 69.0% N At atmospheric pressure and 450 C.using a contact time of 3 seconds, 97.0% of the l-butene was consumed toform 1,3-butadiene in 67.4% conversion.

EXAMPLE 4 1.404 g. Li CO 12.582 g. of Bi 0 and 14.395 g. of M00;, aremixed by grinding in a mortar, calcined in air for 16 hours at 625 C.,reground, and calcined for an additional 16 hours at 600 C. The productis a singlephase tetragonal scheelite-type structure with a formula LiBi lj MoO and with the lattice parameters, a =5.232 A. and c =11.S30 A.A portion of the product was tested as a catalyst for the synthesis ofacrolein under conditions comparable to those in Example 3. Conversionof propylene feed to acrolein was 65.1 mole percent. A similarcomposition Li Bi MoO without cation vacancies converted only 23.7% ofpropylene to acrolein under the same conditions.

EXAMPLE 5 Solutions of 1.1495 g. of metallic Bi in HNO and 0.5946 g. ofAgNO in H O are mixed together and added with stirring to a solution of1.7656 g. of

in H O. The product recovered by evaporation is ground and calcined inair overnight at 500 C., reground and again calcined overnight at 600 C.The final product is a single-phase tetragonal scheelite-type structurewith a formula of Ag Bi MoO and with a =5.284 A. and c =11.678 A.

When tested for the synthesis of acrolein using a 10/1 air/propyleneratio at least 53% conversion to acrolein was obtained whether the feedgas was diluted with nitrogen, diluted with a steam/nitrogen mixture, orused undiluted. With ammonia in the feed this product catalyzedacrylonitrile formation (55% conversion). A similar composition Ag BiMoO containing no cation vacancies gave less than 4% conversion toacrolein under similar conditions.

9 EXAMPLE '6 Bi(NO -H O (136.61 g.) and 37.42 g. of

Fe(NO -9H O are dissolved in dilute nitric acid; 33.70 g. of

(NHQ Mo O -4H O are dissolved in water. The two solutions are mixed andthe pH is adjusted to 6 using NH OH. The solution is then evaporated todryness, ground and heated to 600 C. for about 10 hours. The product hasthe composition has a monoclinic variation of the schellite crystalstructure and is catalytically active for the synthesis of acrylonitrile from propylene, 71% conversion of feed propylene toacrylonitrile.

What is claimed is:

1. A catalyst which has scheelite-type crystal structure of the generalformula l-z z i wherein A represents cations having ionic radii in therange of about 0.9 to about 1.6 A. some of which are trivalent bismuthand others are optionally selected from the group consisting of silver,sodium, lithium, potassium, yttrium and lanthauides;

B represents cations having ionic radii in the range of about 0.3 toabout 0.5 A. comprising molybdenum;

[:1 represents a cation vacancy in the crystal structure; and

z is a positive number up to about 0.15 expressed as 2. A catalystaccording to claim 1 on a support.

3. A catalyst according to claim 1 which includes iron.

4. A catalyst according to claim 1 wherein the A ions include sodium.

5. A catalyst according to claim 1 wherein the A ions include lithium.

6. A catalyst according to claim 1 wherein the A ions include silver.

7. A catalyst according to claim 1 wherein the A ions include yttrium.

8. The catalyst according to claim 1 which is .a2 .5c .m 4

9. The catalyst according to claim 1 which is na js m s 10. The catalystaccording to claim 1 which is .a5 .ss .i 4 11. The catalyst according toclaim 1 which is min :.oo1 .sz'1 .s'1a 4 12. A catalyst according toclaim 1 having the formula il. 221.- III. B 0

References Cited UNITED STATES PATENTS 3,415,886 12/1968 McClellan252-456 X 3,316,182 4/ 1967 McDaniel et a1. 252-467 X 3,380,931 4/1968Ryland 252-467 X FOREIGN PATENTS 967,877 8/1964 Great Britain 252-467967,878 8/ 1964 Great Britain 252-467 DANIEL E. WYMAN, Primary ExaminerW. J. SHINE, Assistant Examiner US. Cl. X.R. 252-467, 470, 463

